1. Field of the Invention
The present invention relates to an interferometer using a vertical-cavity surface-emitting laser (VCSEL), and more particularly, to an interferometer which incorporates a single mode VCSEL to facilitate miniaturization through integration of parts, save manufacturing cost, and improve precision.
2. Description of the Related Art
An interferometer is an optical device utilizing superposition of wave which has a long history. Before the invention of the laser, pseudo-monochromatic light had been produced by transmitting light from a mercury lamp or natural light through a filter in order to create an interference fringe. After the introduction of the laser, the interferometer has been relatively simplified in design and improved in coherence, which makes the interference fringe to be observed relatively easier. Examining a Michelson interferometer for example, this type of interferometer can be composed of one laser, two mirrors and a photodetector or screen.
A laser used as a light source should have a single frequency to obtain an interference fringe that is temporally still. For this purpose, a gas laser such as a He—Ne laser is widely used. In case of a solid laser or semiconductor laser having a wide optical gain bandwidth, a separate technology such as external feedback for filtering in a transverse or longitudinal mode is required to produce a single wavelength. Such an interferometer is generally used for precision length measurement, and particularly, as a location sensor in an event of actuating a precision stage. According to properties of the interferometer, when there is one wavelength difference between two optical paths, the final interference fringe is shifted by one period. Generally, one of the optical paths is fixed but the other one is attached to a movable stage so that the movement of the stage can be measured more precisely.
However, existing interferometers adopt a gas laser, which increases the volume of an optical system. This makes it difficult to miniaturize the location sensor using an interferometer. In case of semiconductor lasers, a distributed Bragg mirror structure can also be applied to an edge emitting laser to realize a light source of an interferometer, acting in a single mode. However, this type of laser is difficult and expensive to fabricate and thus can be rarely mass-produced.